1. Field of the Invention
The present invention relates to a chip type solid electrolytic capacitor, a method for preparing the same, and an anode terminal used in the preparation method.
2. Description of the Related Art
A chip type solid electrolytic capacitor (hereinafter often referred to simply as “chip type capacitor”) is in the form of a chip comprising, as its core, a capacitor element having an anode-dielectric-cathode structure. More specifically, the capacitor element has a structure comprising an anode made of a metal exhibiting valve action (valve metal), an oxidized layer as a dielectric layer formed over the surface of the anode, an electrolyte layer formed on the dielectric layer, and a cathode in the form of an electrically conductive layer formed on the electrolyte layer.
In this connection, the valve metal means a metal capable of forming an oxidized layer whose thickness can be controlled by anodic oxidation. Specifically, valve metal includes niobium (Nb), aluminum (Al), tantalum (Ta), titanium (Ti), hafnium (Hf) and zirconium (Zr). Actually, however, aluminum, tantalum and niobium are mainly used.
With respect to Al, a foil is generally used as the anode, and with respect to Ta and Nb, a porous body prepared by sintering a Ta- or Nb-based powder is used as the anode.
Of those solid electrolytic capacitors, a chip type capacitor comprising a porous sintered body as an anode is particularly adaptable to miniaturization and capable of being adapted to have a high capacity, and hence there is strong demand therefor as a part which meets needs of miniaturization of a cellular phone, information terminal equipment or the like.
In recent years, for further miniaturization, there has frequently been employed such a structure of a chip type capacitor that electrodes (an anode and a cathode) are exposed only in the bottom surface of the chip type capacitor.
Specifically, the structure is different from a theretofore employed structure in which each of the electrodes is exposed from a side surface and led to the bottom surface by bending in that a capacitor element incorporated therein is so disposed as to bridge the anode terminal and the cathode terminal (so-called “face down structure”).
By employing such a structure (face down structure), further miniaturization of a chip type capacitor is realized, and possibility of occurrence of short circuits between the electrodes and other electronic parts mounted around the electrodes is reduced. As a result, packaging density is increased.
A chip type capacitor having such a conventional face down structure and a method for the preparation thereof are disclosed, for example, in Japanese Patent No.3084895 by Sano et al. and in Japanese Unexamined Patent Publication No.2001-267180 by Narita et al. In this preparation method, an anode lead wire of a capacitor element having a capacitor body and the anode lead wire extending from the capacitor body is rested on an anode terminal. The anode terminal has a portion bent to form a connecting portion, and the connecting portion and the anode lead wire are welded together with a laser beam.
When the anode lead wire and the connecting portion of the anode terminal of the capacitor are bonded to each other by the above-mentioned method, a relatively high bond strength can be obtained although the bond is made in a narrow area corresponding to the thickness of the anode terminal.
However, the above-mentioned method does not always provide a sufficient surface area of the bond between the anode lead wire and the anode terminal. Further, the method does not stably provide a sufficient diffusion layer formed including a bonded area after the welding step. For these reasons, capacitors having insufficient bond strength are likely to be yielded.
If the bond strength is insufficient, the anode lead wire and the connecting portion of the anode terminal which have been bonded together are disconnected in a step for mounting the capacitor on a substrate. This is because the mounting step is a heat applying step, and a sheathing resin filled between the element body and the anode terminal is expanded by influence of the heat to exert pushing forces on the anode body of the element body and the connecting portion of the anode terminal in such directions that the anode lead wire and the connecting portion are brought apart from each other. Such a capacitor of which the anode lead wire and the connecting portion that have been bonded together are disconnected is often called an “open defectiveness” capacitor and does not function as a capacitor.
Further, it is difficult to check from an appearance of a bonded area whether a capacitor has an “open defectiveness” or not. Accordingly, presence or absence of an “open defectiveness” has influence on reliability of electronic equipment.